The statements in this section merely provide background information related to the present disclosure and should not be construed as constituting prior art.
Hip arthritis has been found to be a common source of hip pain as the cartilage of the joint begins to wear over time. A layer of cartilage lines the ball and socket of the hip joint and allows the joint to move freely. As the cartilage wears, hip movements can become stiff and painful. If the arthritis becomes severe enough, hip replacement surgery may become necessary.
Hip replacement is a surgical procedure in which the hip joint is replaced by a prosthetic implant. This can be done as a total replacement or a hemi-replacement. In this procedure, a first step is to remove the damaged cartilage and bone. As known to the skilled artisan, the hip joint has two sides comprising a ball (i.e., femoral head) and socket (i.e., acetabulum). To remove the worn out head of the hip joint, the bone is cut and removed. Once removed, the worn out acetabulum, which is part of the pelvis bone, is reamed to scrape away the damaged cartilage and bone. The new socket of the hip replacement can be inserted. To do so, an acetabular component (e.g., “cup”) is inserted into the pelvis area by making the socket slightly smaller than the acetabular component to create a press-fit connection.
Once the acetabular component is attached, the ball is supported by an implant inserted into the hollow center of the femur. This becomes the femoral stem. The center of the thigh bone is shaped to accommodate a tight fit with the femoral stem. The stem can be firmly held in the bone with the use of cement, for example, or alternatively the attachment can be press-fit. With the stem inserted into the femur, the ball of the ball-and-socket hip joint can be inserted on top of the stem. Next, the ball-and-stem portion can be inserted into the socket to complete the hip replacement procedure.
Years ago, this procedure was performed with the ball and stem being pre-fabricated as a single unit. However, over time, the two components became modular so different sizes of ball and stems could be made. The ball (or head) was shaped with a tapered bore through which a similarly shaped tapered stem was coupled. With there being modularity between the parts, concerns arose about possible corrosion at the modular interface. As is known, corrosion can occur at the interface between two metallic components formed of different material, especially when the two components rub against one another. To reduce corrosion, the stem was formed of titanium and the head of cobalt chrome, both of which are mostly benign and have been found to be safe in vivo even when in intimate contact. Each material has an inherent passive layer, and generally goes through a known passivation process to prevent corrosion.
Through different studies, it has been found that if a larger sized ball is coupled to a stem, patients can experience better range of motion. This has been found to be important as hip replacement patients are younger and more active following surgery. Another benefit of using a larger sized ball is a reduction in the number of dislocations. So, as the size of the stem remained constant, the diameter of the ball began increasing from 22 mm to 56 mm or greater in some orthopedic implants. However, as the ball size increased, the torque acting on the ball and the tapered stem increased causing movement between the ball and stem. This movement, referred to as micro-motion, began causing fretting corrosion at the tapered injunction as the passive outer layer of material broke down. Liquid, such as bodily fluid, is able to penetrate the injunction and cause the corrosion. Outer barrier layers have not worked at the injunction as these materials tend to break apart in a high stress environment.
As a result, a need exists for a seal to reduce or prevent the penetration of bodily fluids at the taper injunction of orthopedic implants. It is desirable to position the seal at the injunction to prevent, or at least delay, the onset of corrosion at the injunction.